The invention relates to planetary gear transmissions and components, but more particularly, the invention relates to a belt drive differential where a planet pinion engages a flexible belt.
In a belt drive differential, planet pinions in the form of pulleys are rotatably mounted to a carrier and orbited in a circular path about a carrier axis. Output pulleys arranged substantially coaxial with the carrier axis transmit power to shafting.
In a belt drive differential, two or more planet pulleys are rotatably mounted to a carrier and orbited in a circular path with the carrier about a carrier axis. Two output pulleys are arranged coaxial with the carrier axis and a belt bent in a circuitous path is entrained around and engages the planet pulleys and output pulleys which are in different planes. Such a drive with multi-plane pulleys is commonly known as a mule pulley drive.
In the belt differential disclosed in U.S. Pat. No. 3,543,608 to Meihak, coaxial planet V-pulleys are used in conjunction with a V-belt and output V-pulleys individually fixed to and near ends of separate shafts that are coaxial with the carrier axis. Ends of the two shafts are commonly journaled to the carrier and require additional journaling with thrust capability to prevent axial separation of the commonly journaled shaft ends and the fixed output V-pulleys. The position of the planet V-pulleys require periodic adjustments for "belt take up" by means of threaded fasteners to adjust belt tension and to compensate for belt wear. Summarily, the V-belt drive has limited power capability, requires intermittent belt take-up adjustments which has the effect of changing belt drive length to minimize V-belt slippage; and two shafts are required, one fixed to each of the two output pulleys.
In the belt differential disclosed in U.S. Pat. No. 5,445,572 to Parker, both non-coaxial frustoconcial and cylindrical planet pulleys are used in conjunction with a toothed synchronous belt and output sprockets individually fixed near ends of separate tubular gear shafts that are coaxial with the carrier axis and are journaled over a central or third coaxial shaft. The two tubular shafts require additional journaling with thrust capacity to prevent separation of the tubular shaft ends with fixed output sprockets. The position of the planet frustoconical pulleys and cylindrical pulleys require belt take-up adjustment for belt drive length to tension the belt to impede it from ratcheting on the output sprockets. A second set of frustoconical guide pulleys are also required juxtaposed the belt and opposite planet pulleys to help guide the belt and impede belt ratcheting. Summarily, the synchronous belt drive requires adjustment of the belt drive length to minimize belt ratcheting on the output sprockets; frustoconical rollers are required to impede belt movement that can cause belt ratcheting; and three shafts are required, one fixed to each of the tubular gear shafts and, a central shaft.
There are several major problems with the aforementioned belt drives. A major problem is that there is not a fixed or constant belt drive length. The position of the planet pulleys must be re-adjusted for belt take-up to obtain or change to a belt drive length where V-belt slipping or synchronous belt ratcheting is impeded.
Another problem with the prior art belt drive differentials is associated with the use of multiple output shafts. The output pulleys or sprockets are each connected to separate shafts. Special journaling with thrust capability is required to keep the output sprockets and pulleys in a position that does not impact belt drive length. The use of a third or central shaft introduces a problem of lubrication and fretting corrosion with the journaled tubular shafts.
A particular problem with the Parker '572 belt drive differential is that it uses planet pulleys as idlers that engage the tips of belt teeth to adjust belt drive length and define a curved surface about which belt bending is forced. The pulleys press against the tips of the belt teeth allowing the belt tensile member (i.e., cord) to bend at very small bending radii in areas between belt teeth which causes crimping of the belt's tensile member that is deleterious to belt life. The smaller the radius of the pulley, the shorter the belt life. The use of frustoconical pulleys is particularly deleterious to a cord tensile member because the belt must bend over various radii across its width as it presses against the frustoconical pulley.
A belt drive length is associated with the length of the belt's tensile member because belt bending takes place at the tensile member. Consequently, bending of a synchronous belt and its belt drive length cannot be precisely controlled by small diameter planet pulleys that press against tips of belt teeth.